Simple Volt Clamp: where to get components?

Discussion in 'The Projects Forum' started by DasValdez, May 18, 2006.

  1. DasValdez

    Thread Starter New Member

    May 18, 2006
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    My background is computer programming, so I can figure out the logic part, and I can research topics to figure out how to build simple circuits... but once I've gotten close to what I need to create I have no clue where to buy the parts. I've done some googling but have no need to purchase a stack of 1000 zeners, and the sites I've found which sell single items/low qty I'm having trouble finding what I need.



    My circuit is simple, automotive application, sensor outputs 0-5v normally. Under certain circumstances (say when the motor is producing 3 times the power it came from the factory with), the sensor might go above 5v... to 6 or 10v. If it goes any higher than 5v, it creates a fault code, which I need to get rid of.

    Below 5v, I want to Vout whatever my Vin is.

    At something below 5v, I want to clamp the Vout to a set level, say 4.5 or 4.75v.

    It seems easy to do with a 1kO resistor and zener diode of the right size and orientation... from what I've read I'd need a 4.75v diode or whatever level I wanted to clamp at.

    The resistor I can find, but the correct V zener I've having trouble locating.

    Any resources to nab a few for my project would be greatly appreciated.

    TIA-

    Val
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    [attachmentid=1434][attachmentid=1432]
    This may be what you are looking for. The transistor should have a gain of at least 100. Collector and emitter are connected across the supply you wish to clamp.

    The two resistors will pass current until there is .7v across the 10K (4.62V across them both). Then the transistor will conduct preventing the supply voltage from rising above 4.62

    Note that there will always be a load of 66K on your circuit.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Newark

    Six cents each, plus shipping.
     
  4. DasValdez

    Thread Starter New Member

    May 18, 2006
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    Excellent, thanks for the link!



    Which circuit would be the recommended method for solving my problem? It seems the general answer I find for a voltage clamp when I search is the zener setup, I haven't seen it solved as pebe posted above.

    Are there any benefits or drawbacks to either design? Will one spike past peak then settle back to the clamp voltage, or will one act as more of a "soft" clamp than a "hard" one... (maybe something like a Vout that goes logarithmic as it gets close to Vclamp?)

    Either one should suit my needs, just curious as to whether there's a more simple solution than what I've found.

    Thanks again!
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    When you mentioned an automotive application, I thought of power spikes from the motor containing a lot of energy. That's why I proposed the transistor approach as probably being 'beefier' than a zener. In fact, the Vishay zener that Thingmaker pointed to has a dissipation of 200mW compared to 500mW of a 2n2222, so there's not much in it. In the circumstances, I would go for the zener.

    A zener wired across the supply will hard clamp at its zener voltage, ie. it sits there and does nothing until 4.7V is reached then conducts enough to clamp the voltage.

    Putting a resistor in series will create a soft clamp because when the zener conducts the current must pass through the resistor and this will allow the supply to increase above the zener voltage. To be effective, the resistor needs to be much smaller than the supply impedance.

    I hope that helps.
     
  6. DasValdez

    Thread Starter New Member

    May 18, 2006
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    It does indeed hep, thank you for the explanation!

    Now the only question is... where can I get a 4.5/4.75V zener without a 37 day lead time (as per the Vishay site).

    I can't fathom waiting 37 days for a .06 part... I'll probably order 100 of them just so I don't feel silly.

    Hrmm, I might even feel silly @ $6 for the total order. lol

     
  7. DasValdez

    Thread Starter New Member

    May 18, 2006
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    0
    dbl post deleted
     
  8. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    I dont know which country you are in but in the UK there are many suppliers giving 24Hr delivery.

    Try Googling for 'electronic component suppliers'. I got millions of hits.
     
  9. DasValdez

    Thread Starter New Member

    May 18, 2006
    4
    0

    You are correct, that search term hit the spot. I'd been doing silly searches like "zener diode" and got a lot of tech/schematic type results but not a lot of suppliers where I could figure out what I was looking at.

    Thanks again...
     
  10. DasValdez

    Thread Starter New Member

    May 18, 2006
    4
    0
    I am the master of 2x postine
     
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